Vaesen limits the consideration of the motor component of the tool use to two aspects: eye-hand coordination and adaptation of the body schema. However, recent findings show that cognitive processes involved in control of limb movements may be much more complex and diverse. In particular, the leading joint hypothesis developed in my lab predicts extensive and sophisticated cognitive processes involved in control of human limb movements. Here I discuss the predicted processes and their relevance to the uniqueness of human tool use.
The leading joint hypothesis suggests that movements are organized by exploiting the multi-joint structure of the limbs (for reviews, see Dounskaia Reference Dounskaia2005; Reference Dounskaia2010). The multi-joint limbs consist of chains of approximately rigid segments brought in motion by muscles spanning the joints. During motion, the segments mechanically interact with each other. The leading joint hypothesis suggests that the nervous system benefits from these interactions and exploits them for movement production by introducing a hierarchy among the joints. One (leading) joint is used to generate energy for the entire limb motion, similar to the whip handle that brings in motion the entire whip. Another analogy for the role of the leading joint is towing one vehicle with another. The role of the musculature at the other (subordinate) joints is to modify their passive motion and adjust it to task requirements. An analogy for subordinate joint control is steering the towed vehicle to correct deviations of it from the direction followed by the towing vehicle. Thus, the leading joint generates limb motion and the subordinate joints regulate this motion according to the demands of the task.
The leading joint hypothesis implies that humans use mechanical properties of the limbs to achieve everyday goals in the same way as they use properties of tools. This interpretation suggests that the uniqueness of humans known for tool use may, in part, be observed in limb movements. One can argue that since primates have a similar multi-joint structure of the limbs, the leading joint hypothesis predicts similarity of cognitive processes underlying movement control in humans and primates. This inference may be only partially correct. Indeed, the leading joint hypothesis implies two components of movement planning, each of which may be a source of differences in motor control between humans and primates. The first component is the selection of a leading joint and planning its motion that, after modification at the subordinate joints, can perform the task. The second component is the determination of the subordinate joint control that will adjust the limb movement to the task requirements. Although these two components of movement planning must be present in both humans and primates, the processes involved in human movement control may be more sophisticated.
The superior ability of humans to use leading joint motion for movement production is evident from the vast repertoire of human motor actions. The variety of movements performed during sports activities, dancing, and expressive gestures demonstrates exclusive creativity of humans in exploiting mechanical effects that can be generated within the body through different leading joint motions. This creativity may be a crucial component in mastering diverse tools because each tool changes mechanical properties of the limb in a specific way, and the leading joint motion needs to correspond to these changes.
The subordinate joint control is another possible source of differences in motor performance between humans and primates. Human motor control may be characterized by greater diversity of the ways in which the subordinate joint musculature can modify passive motion caused by the leading joint. This prediction finds support in our recent findings that the modification of passive motion of the subordinate joints requires substantial neural resources. In our experiments, participants performed a free-stroke drawing task that provided freedom in the selection of movement direction (Dounskaia et al. Reference Dounskaia, Goble and Wang2011; Goble et al. Reference Goble, Zhang, Shimansky, Sharma and Dounskaia2007). Arm movements were performed through flexion and extension of the shoulder and elbow. Participants demonstrated consistent directional preferences by frequently selecting certain movement directions and strongly avoiding some other directions. The most preferred directions were those in which the subordinate joint moved largely passively. The most avoided directions were those in which the subordinate joint had to rotate in the direction opposite to the passive rotation. Cognitive load created by a secondary task (counting back by 3's from a given number, e.g., “57, 54, 51, …,” during the performance of the primary, free-stroke drawing task) markedly strengthened the directional preferences (Dounskaia & Goble Reference Dounskaia and Goble2011). These results show that the modification of passive motion at the subordinate joints requires substantial cognitive effort and that humans tend to avoid this effort. It is plausible that this tendency is stronger in primates, who may have limited ability to provide substantial modifications of passive motion at the subordinate joints. However, the capability to accurately modulate passive mechanical effects with muscle activity may be crucial for sophisticated tool use.
To summarize, the interpretation of multi-joint movement control offered by the leading joint hypothesis provides new insights with respect to complexity of cognitive processes involved in motor performance. The idea that the limbs are used as tools for achieving goals of daily life suggests that the uniqueness of humans in tool use may be not limited to the higher levels but presented already at the level of motor control . Obtaining solid support for this hypothesis is a subject for future research.
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Vaesen limits the consideration of the motor component of the tool use to two aspects: eye-hand coordination and adaptation of the body schema. However, recent findings show that cognitive processes involved in control of limb movements may be much more complex and diverse. In particular, the leading joint hypothesis developed in my lab predicts extensive and sophisticated cognitive processes involved in control of human limb movements. Here I discuss the predicted processes and their relevance to the uniqueness of human tool use.
The leading joint hypothesis suggests that movements are organized by exploiting the multi-joint structure of the limbs (for reviews, see Dounskaia Reference Dounskaia2005; Reference Dounskaia2010). The multi-joint limbs consist of chains of approximately rigid segments brought in motion by muscles spanning the joints. During motion, the segments mechanically interact with each other. The leading joint hypothesis suggests that the nervous system benefits from these interactions and exploits them for movement production by introducing a hierarchy among the joints. One (leading) joint is used to generate energy for the entire limb motion, similar to the whip handle that brings in motion the entire whip. Another analogy for the role of the leading joint is towing one vehicle with another. The role of the musculature at the other (subordinate) joints is to modify their passive motion and adjust it to task requirements. An analogy for subordinate joint control is steering the towed vehicle to correct deviations of it from the direction followed by the towing vehicle. Thus, the leading joint generates limb motion and the subordinate joints regulate this motion according to the demands of the task.
The leading joint hypothesis implies that humans use mechanical properties of the limbs to achieve everyday goals in the same way as they use properties of tools. This interpretation suggests that the uniqueness of humans known for tool use may, in part, be observed in limb movements. One can argue that since primates have a similar multi-joint structure of the limbs, the leading joint hypothesis predicts similarity of cognitive processes underlying movement control in humans and primates. This inference may be only partially correct. Indeed, the leading joint hypothesis implies two components of movement planning, each of which may be a source of differences in motor control between humans and primates. The first component is the selection of a leading joint and planning its motion that, after modification at the subordinate joints, can perform the task. The second component is the determination of the subordinate joint control that will adjust the limb movement to the task requirements. Although these two components of movement planning must be present in both humans and primates, the processes involved in human movement control may be more sophisticated.
The superior ability of humans to use leading joint motion for movement production is evident from the vast repertoire of human motor actions. The variety of movements performed during sports activities, dancing, and expressive gestures demonstrates exclusive creativity of humans in exploiting mechanical effects that can be generated within the body through different leading joint motions. This creativity may be a crucial component in mastering diverse tools because each tool changes mechanical properties of the limb in a specific way, and the leading joint motion needs to correspond to these changes.
The subordinate joint control is another possible source of differences in motor performance between humans and primates. Human motor control may be characterized by greater diversity of the ways in which the subordinate joint musculature can modify passive motion caused by the leading joint. This prediction finds support in our recent findings that the modification of passive motion of the subordinate joints requires substantial neural resources. In our experiments, participants performed a free-stroke drawing task that provided freedom in the selection of movement direction (Dounskaia et al. Reference Dounskaia, Goble and Wang2011; Goble et al. Reference Goble, Zhang, Shimansky, Sharma and Dounskaia2007). Arm movements were performed through flexion and extension of the shoulder and elbow. Participants demonstrated consistent directional preferences by frequently selecting certain movement directions and strongly avoiding some other directions. The most preferred directions were those in which the subordinate joint moved largely passively. The most avoided directions were those in which the subordinate joint had to rotate in the direction opposite to the passive rotation. Cognitive load created by a secondary task (counting back by 3's from a given number, e.g., “57, 54, 51, …,” during the performance of the primary, free-stroke drawing task) markedly strengthened the directional preferences (Dounskaia & Goble Reference Dounskaia and Goble2011). These results show that the modification of passive motion at the subordinate joints requires substantial cognitive effort and that humans tend to avoid this effort. It is plausible that this tendency is stronger in primates, who may have limited ability to provide substantial modifications of passive motion at the subordinate joints. However, the capability to accurately modulate passive mechanical effects with muscle activity may be crucial for sophisticated tool use.
To summarize, the interpretation of multi-joint movement control offered by the leading joint hypothesis provides new insights with respect to complexity of cognitive processes involved in motor performance. The idea that the limbs are used as tools for achieving goals of daily life suggests that the uniqueness of humans in tool use may be not limited to the higher levels but presented already at the level of motor control . Obtaining solid support for this hypothesis is a subject for future research.